1. Field of the Invention
The invention concerns an arrangement and a method for active vibration damping of an x-ray radiator with a counter-vibration acting on the x-ray radiator.
2. Description of the Prior Art
Two-dimensional or three-dimensional images of an examination region of a patient for diagnosis or therapy purposes are generated with tomography apparatuses. For example, three-dimensional slice images are generated with a computed tomography apparatus. The basic design of a computed tomography apparatus includes a gantry (support portal) with a stationary support frame in which a rotating frame is mounted such that it can rotate around an axis. An acquisition system that has an x-ray radiator and a detector arranged opposite the x-ray radiator is arranged on the rotating frame. Projections are acquired along a spiral path from a number of different projection directions by rotation of the rotating frame with simultaneous, continuous feed of a patient borne on a bed device in the direction of the system axis. Since 99% of the electrical energy used in the generation of x-ray radiation of the x-ray radiator is converted into thermal energy, the computed tomography apparatus has a cooling device in order to avoid an overheating of the electronic components.
In the operation of the mechanical and electrical components of the tomography apparatus, a noise level arises that is perceived as uncomfortable both by the patient and by the operator. For example, interfering structure-borne sound and airborne sound waves arise due to the rotation of the rotating frame, and/or due to the rotation of the anode within the x-ray radiator, and/or due to the operation of the cooling device. A resonance body that amplifies the noise amplitudes is formed by the casing (having) of the x-ray radiator. The patient is exposed to a particularly high noise level especially in the tunnel opening of the gantry through which the patient is shifted during the scanning.
An important aspect in the design of a tomography apparatus is therefore minimization of the interfering noise arising during operation of the tomography apparatus. There are two different approaches in order to suppress or minimize the propagation of interfering structure-borne and airborne sound waves in a tomography apparatus. One possibility is to reduce the interfering noise generation directly by optimization of the components causing the noise. For example, it would be possible to minimize the structure-borne sound wave propagation due to rotation of the rotating frame or rotation of the anode by the use of a noise-optimized rotating bearing. Such an optimization, however, is associated with high cost, and the achieved reduction of the interfering noise is normally insufficient.
Another possibility is to suppress the sound propagation by the use of noise damping mats. For example, the noise damping mats are adhered on the inside of the housing of the tomography apparatus to reduce the airborne noise propagation. The structure-borne sound propagation can also be minimized by use of corresponding passive damping materials at the contact points provided for mounting of the components. However, an effective reduction of the noise propagation is achieved only with a defined thickness of the damping material. The structural volume available for such damping is very limited, such that these measures are normally not sufficient in order to reduce the interfering noise to a desired level.
A tomography apparatus with a noise cancellation device and a method to reduce an interfering noise arising during operation of the tomography apparatus are specified in DE 102008047814 A1. The noise cancellation device has a control unit to provide a noise cancellation signal and a sound generation unit to convert the noise cancellation signal into an anti-noise signal that is shifted in terms of its phase by 180 degrees relative to the interfering noise. In this way the interfering noise can be effectively reduced during operation of the tomography apparatus.
Published Application US 2005/0281391 discloses a computed tomograph in which a vibration of the support frame is determined and, due to the oscillation, measures to remedy it are implemented.